I have a 7000 Ford diesel in the shop that had the oil cooler leaking into the radiator. It has a lot of goo in the bottom of the water jacket. I had to split it to replace a leaking frost plug on the back of the block and found the goo when I drained the water. The engine is still together with the front axle, so vatting it isn't an option.
Lots and lots of flushing with hot water and soap. Seems like you're never going to get it. Be prepared for a couple of coolant changes even after you think you've got it and put it back together and run it for a while. There are many, many little pockets and areas that have to give up the oil trapped in there. It will get clean eventually.
Use a soap made for automatic dish washers, low foam. Like Shimmy says it won't happen the first try might take several. Normally I would not suggest running an engine without the thermostat but in this case might help some by keeping the flow going fast, just keep eye on tempgauge.
I 2nd kshansen. I was told by a lab that delt with coolant that automatic dish soap is the best thing to use.
cascade powder soap. flush it with soap a couple times and then with straight water until the water comes out clean, running the engine up to operating temp each time.
Had the same freeze plug issue with our Ford 7710 years ago . Had to split the tractor as well . What had rusted the freeze plug was a black muddy / gritty goo that was in the bottom of the water jacket . Seemed like there was quite a bit of the nasty stuff . Almost like they did not get all the sand out after casting the block ? We pulled all the freeze plugs & block heating elements and used a power washer & dish soap to clean it all out . Had another episode years later . Developed a pin hole in the block around the cam shaft area , filled the crankcase with anti freeze , that was cute . I had to pull the engine on this round but was able to repair the leak buy drilling & tapping the pin hole and installing a set screw .
Remove thermostat put spout back on add muck out flush tecktalloy do a good one run till about 80 deg center grade stop eng pop radiator hoses off so u can flush radiator and block separately now make a fitting with one in being water and the outher air hook up flush with water first when the radiator is full of water let the air go the air will push the water out taking a lot of crap with it repeat until clean then do it a few more times jots to make sure repeat the proses with the block
It depends where you're located, and how cold it gets. I do agree with most, automatic dish soap and water about four five times.
got any pics of the 7000 in one piece?......classic, classic tractor that